You need to confirm the converter is getting 120v input. It might not if it is plugged into an outlet that has its GFCI popped on that outlet circuit. Or the breaker panel with the breaker marked "converter" might be snapped over.
Once you are sure there is 120v to the converter, use your multimeter to see if it has 13.6v at its DC terminals. If not, it is toast.
If no multimeter, disconnect that 12v battery by taking the wires off one of its posts. Do the 12v lights in the RV work? If so the converter has DC output.